As has been noted in several technical journals recently, atherosclerosis is a leading cause of non-accidental death in the United States. Because the two conventional treatments, bypass surgery and balloon angioplasty, have known disadvantages, laser angioplasty is being turned to as a more effective therapeutic technique.
Even laser angioplasty has its problems, however. Chief of these is the inability to quickly distinguish between "friend" and "foe", that is, between healthy vascular tissue and the diseased plaque. Conventionally, transluminal illumination is delivered to and subsequently received from tissues, such as by diffuse reflection or direct transmission, to identify and distinguish plaque from healthy tissue. The same optics are then used to deliver therapeutic laser light to ablate diseased tissue. In the past, illumination has used light in the near UV region, and the detection has been based upon fluorescence. In some cases, the detected fluorescence was of certain fluorophors claimed to exist in the plaque. Still others are said to detect the natural fluorescence of both plaque and healthy tissue, and compare the signal with predetermined values that are said to represent healthy tissue or plaque. Such a system is described in U.S. Pat. No. 4,718,417 and in Lasers in Surgery and Medicine, Vol. 9, page 109-116 (1989). The latter describes the following equation to determine a so-called LIF ratio for the comparison: LIF=Co+C.sub.1 I.sub.F (.lambda..sub.1)+ . . . +C.sub.13 I.sub.F (.lambda..sub.13), using 13 different wavelengths to detect the fluorescence.
In either case, there are disadvantages in such prior systems. First of all, the illumination uses near UV light, which has very low penetration power of slightly less than 100 .mu.m thickness, "Optics of Human Skin", Journal of Investigative Dermatology, Vol. 77, p. 13-19 (1981). To avoid ablating the wrong (healthy) tissue, such detection techniques require the laser to also ablate only on that order. A thin layer of ablation means that the total removal time is prolonged, and more illumination and detection, and thus delay, is required to remove the same amount of diseased tissue. In any event, delays while double-checking the site cannot be easily skipped--failure to identify tissue as healthy before ablating it with laser energy can lead to vessel perforation. The risk of such perforation is considered to be "the major impediment to safe and effective laser angioplasty" in current technology, Applied Optics, Vol. 27, No. 9, p. 1844 (May 1988). The delay just mentioned is further aggravated when using fluorescence for detection--laser ablation light tends to dehydrate remaining plaque, which in turn can quench the fluorophore. To avoid this, an operation may have to wait up to 10-20 minutes for sufficient rehydration to occur. Such a wait is intolerable, when several ablative "blasts" are needed to get through the plaque 100 .mu.m at a time.
Second, the inability to detect deep tissues has a further problem in that it cannot detect, until it is too late, that all of the vascular wall at a particular point has been converted to diseased tissue. That is, even the media tissue may be diseased, leaving only adventitia underneath it. That however can have a thickness of only 100 .mu.m. Since the ablation removal conventionally is up to about 100 .mu.m, the system that can "see" only less than 100 .mu.m may end up seeing 80 .mu.m of plaque and 20 .mu.m of adventitia, identify it as primarily plaque, and undesirably ablate away all but 80 .mu.m of the vascular wall.
Third, another difficulty exists in relying on fluorescence of plaque--not all plaque is homogeneous; nor do all diseased vascular tissues fluoresce. In fact, plaque has been found to contain some or all of the following: lipids, connective tissues, thrombus, necrotic tissues, mineralized deposits, smooth muscle cell tissues which have proliferated from the vessel wall, as well as other constituents. Therefore, it is very difficult to be sure the fluorescence is complete enough and specific enough to permit the simultaneous multicomponent analysis capability that is an essential prerequisite of an effective and safe "smart" laser angioplasty system. That is, many plaque constituents "look like" other healthy body tissues. For example; platelets that deposit onto the surface of an artery (such as in response to a small, naturally occurring injury in the arterial wall) can exude growth factors which will cause the healthy smooth muscle tissues of the vascular wall to grow into the arterial lumen. Such occlusive tissues are likely to look no different from those smooth muscle tissues which make up the media--the thickest element of the vascular wall. Because healthy tissues can appear in "unhealthy" states, such "diseased" tissues are very difficult to identify and discriminate from similar healthy tissues by using fluorescence.
Fourth, there occasionally arise some conditions that produce a thin surface film such as lipid deposits on the healthy tissue that appear to be plaque, but in fact are not because underneath the very thin film (about 50-100 .mu.m) is healthy tissue. A discrimination system using near UV illumination and fluorescence detection can very easily misinterpret such conditions, leading to dangerous attack on healthy tissue. Such attacks can produce acute responses such as a clot. An example of such a condition follows hereinafter.
Examples of prior art patents using the near UV illumination and detection of fluorescence as described above, also include U.S. Pat. No. 4,641,650. Some such prior art techniques rely on the addition of a dye to preferentially "mark" plaque. However, dyes are inherently a systemic foreign agent, subject to risk and governmental regulation. Accordingly, dyes are to be avoided if possible.
Non-UV laser light has been used to examine tissue to identify abnormal conditions. For example, U.S. Pat. No. 4,449,535 teaches the use of a dye laser operating at a wavelength of 805 nm, the region of the very-near-infrared. The difficulty with that kind of detection system is two-fold: i) the wavelength of 805 nm is incapable of detecting two key materials of plaque in blood vessels, namely cholesterol esters and calcification, and ii) dyes for such dye lasers are not suitable for operating in wavelengths determined by the instant invention to be more appropriate for cholesterol esters and calcification.
Yet another example of the use of non-UV light for disease detection is described in U.S. Pat. No. 4,509,522. This describes the use of mid-infrared lasers operating at 5130 nm, to detect the absorption band of carbon monoxide (column 2, line 39). Such radiation is said to be carried over a fiber optic, if the exposure occurs at locations remote from the skin. This technique also suffers two disadvantages: i) the 5130 nm wavelength, although suitable for CO, is not suitable for the detection of the primary plaque components in atherosclerosis (cholesterol esters and calcification); and ii) there is no non-toxic fiber optic known to man that will transmit 5130 nm radiation.
Thus, prior to this invention there has been a need for a quick, yet accurate laser angioplasty instrument that allows for immediate identification of diseased tissue from healthy tissues before firing the laser, particularly such instrument capable of treating atherosclerosis. Such an instrument is desired for its ability to identify and discriminate between all types of diseased and non-diseased tissues, of which atherosclerosis is but one type.